This invention relates to the field of phase shifting circuits for processing electrical signals. It has particular application to cellular communications systems and in particular to apparatus and method for programmably phase shifting a supervisory audio tone in such a system.
Cellular communications systems are growing in popularity and complexity. A cellular communications system comprises a plurality of cells each cell including at least one master station which is in communication with any number of mobile stations within the cell (typically via a fixed remotely controlled transceiver site within or adjacent the cell). A mobile station within the cell communicates to the master station which transfers the communication to conventional land telephone lines for routing the communication to the desired destination. As a mobile station moves from one cell to another, it must cease communication with the remote station of the cell from which it is departing and establish communication with a similar station in the cell which it is entering. In order to synchronize communications between the various stations and the mobile station, the fixed station emits a Supervisory Audio Tone (SAT). The mobile station must be capable of receiving the SAT and retransmitting this tone to the fixed station such that the retransmitted tone has a phase error of less than or equal to plus or minus 20 degrees.
Since each mobile station contains a transmitter-receiver which may impart a different value of phase error to any transmission, and since the mobile stations move with a speed which may affect the phase error of the retransmitted tone, each mobile station must be capable of adjusting the phase of the retransmitted tone in order to produce a retransmitted tone with a phase error of less than or equal to plus or minus 20 degrees with respect to the SAT.
Therefore, a need exists to provide circuitry in each mobile station which can adjustably phase shift the retransmitted audio tone. Moreover, to mass produce such a circuit each one must be capable of providing a different phase adjustment corresponding to the particular radio equipment in each mobile station.
Circuits are known which can phase shift the audio tone by analog means. Such circuitry is necessarily large and expensive to manufacture. This large circuitry takes up a great deal of room in the mobile station thus increasing the cost of the mobile station and making it more difficult to place the mobile station conveniently inside a vehicle.
Thus a need exists for an audio tone phase shift circuit which uses mostly digital circuits, could be manufactured at a reduced cost and in a miniaturized package. Such a digital circuit would have to be adjustable in order to provide the various phase shift adjustments required in different mobile stations. A programmable digital phase shift circuit capable of being integrated into integrated circuits would be most advantageous--and such a circuit is provided by this invention.